Simon,
Workflow was never quite done as envisioned. As with a lot of other ACS 4 things, the plug was pulled. And, also consistent with the rest of ACS 4, I grossly underestimated the amount of effort it would take to finish it the way I'd envisioned.
So that's probably why you find it lacking in many respects. Because it is.
Your thinking is correct, that it shouldn't break (there might be a graphviz issue, did you install graphviz or instruct it to not use graphviz?).
I'll be happy to bounce ideas back and forth and tell you all about the intentions that went into the design, as well as the ideas I have for where I think it should go.
